This invention relates to radio receiver apparatus and more particularly to a radio receiver employing a zero intermediate frequency (IF) design.
Zero IF type receivers are well known in the prior art and essentially a zero IF type receiver skips the step of going to an IF frequency and instead converts the desired incoming signal directly to baseband in a single operation. In addition to the economy of direct conversion there are performance advantages particularly for multiple function radio receivers. To date, the zero IF type receiver has found only limited use, primarily due to the problems resulting from DC offsets present at the output of balanced mixer circuits which are caused by imperfections in their performance.
In any event, as indicated above, the zero IF approach has been widely described in the prior art. U.S. Pat. No. 4,238,850 issued on Dec. 9, 1980 to I. W. Vance and is entitled Transmitter/Receiver for Single Channel Duplex Communication System describes a transmitter receiver which can be used for a cordless telephone. The receiver produces direct conversion from the radio frequency. The local oscillator frequency is modulated by an audio signal to be sent, the modulated signal being passed through a dual splitting and combining network. As described in this patent, the receiver makes use of the zero IF or direct conversion method of demodulation. Such a method is also described in British specification Ser. No. 1530602 published Nov. 1, 1978 to I. A. W. Vance.
U.S. Pat. No. 4,470,147 entitled Radio Receiver with Quadrature Demodulation and Digital Processing issued on Sept. 4, 1984 to J. K. Goatcher and is assigned to the International Standard Electric Corp. This patent depicts a radio receiver which divides the radio frequency input into two channels and in each channel mixes it with the carrier wave frequency. A quadrature shift in a mixed carrier wave frequency enables baseband signals to be filtered from the mixer outputs.
Essentially, the analog quadrature signals are converted to digital form and processed digitally to reproduce the original modulating signal for AM/FM or PM transmissions. For single sideband modulation, the mixing frequency is the sideband center frequency instead of the carrier frequency.
U.S. Pat. No. 4,476,585 entitled Baseband Demodulator for FM Signals issued on Oct. 9, 1984 to J. Reed and is assigned to the International Telephone and Telegraph Corp. This patent shows a modulator which is employed in the zero IF system and uses a local oscillator for providing quadrature output signals at the center frequency of an FM signal to be demodulated. The demodulator has first and second mixers for separately mixing the FM signal with the quadrature signal to provide a first and second output signal, each in quadrature at the outputs of the mixers. These signals are lowpass filtered. A demodulator shown which includes third and fourth mixers with each mixer receiving at an input the output of one lowpass filter. At another input, the mixers receive the third and fourth signals. The third and fourth signals are derived from mixing a variable controlled oscillator signal with a local oscillator quadrature signal.
Essentially, the patent also shows the switching circuit which operates to alternate the third and fourth signals as applied to the input of the third and fourth mixer as well as the output as applied to the difference amplifier and uses an additional amplifier coupled to the outputs of the mixers so that one can utilize automatic gain control in a Zero IF system.
Hence as indicated, the prior art is fully cognizant of the zero IF technique and is further evidence by other U.S. Pat. Nos. which are as follows. U.S. Pat. No. 4,540,958, U.S. Pat. No. 4,476,585, U.S. Pat. No. 4,480,327, U.S. Pat. No. 4,462,107, U.S. Pat. No. 4,488,064, U.S. Pat. No. 4,506,262, U.S. Pat. No. 4,521,892, U.S. Pat. No. 4,525,835, U.S. Pat. No. 4,322,851 and U.S. Pat. No. 4,254,503.
Essentially, the main purpose of such a system is to provide a communications system wherein the amount of tuned circuitry employed is substantially reduced. In obtaining a reduction in the number of tuned circuitry, one is therefore able to integrate large portions of the receiver and produce radio receivers which are extremely compact and reliable. These receivers as indicated may be employed in many areas such as in selective paging systems and so on. Thus to achieve such advantages, the design of such receivers is implemented according to the zero IF technique.
As indicated in such a system, there is present a receiver in which the local oscillator signals are in phase quadrature at the carrier frequency and are each separately mixed with the incoming audio modulated signals. The resultant signals have a zero IF with the two sidebands folded over on each other at the baseband and extending in frequency from DC to the single sideband wave of the original signal. As one can see from the above cited references, the concentration has mainly been in regard to implementing receiver design or a transceiver design utilizing baseband circuitry.
As indicated, baseband circuitry operates at frequencies where integrated circuit technology is well established, and therefore a transceiver which consists primarily of integrated circuits is relatively small and inexpensive. It is, of course, a desire of the prior art to obtain improved performances from the receiver. A major difficulty which arises with conventional zero IF receivers architecture revolves around the DC offsets which are present at the output of the quadrature mixers. These offsets, on the order of millivolts, in the presence of generally much smaller desired signals, causes the low noise baseband amplifiers to saturate before the desired signals could be amplified efficiently to interface with the analog-to-digital converters. One potential method of overcoming this problem is to AC couple the output of the channel filter into the baseband amplifier. This introduces another problem, however, since the DC notch will result in poor transient performance in the presence of pulsed jammers. For some signal modulation types such as DSB (amplitude modulation, double sideband, suppressed carrier), broadening the notch width improves the transient response duration without degrading performance but for many other formats (such as FM and SSB) this results in distortaion, since the notch frequencies contain signal information.
A similar situation occurs for the case of frequency hopping since the DC offsets are different for each frequency. When the hopping rate is fast, this results in significant AC components in the offsets which require a broadening of the notch cutoff frequency. In fact, the notch cutoff frequency may have to be broadened to the point where significant signal distortaion can occur. Hence, as one can ascertain, although there are many good reasons for utilizing the zero IF receiver, there are difficulties in regard to the performance of the same.
It is, therefore, an object of the present invention to provide an improved zero IF receiver which circumvents many of the problems of prior art devices.